POCl3 is a major commercial phosphorylating agent used to synthesize numerous products including biologicals. It has been long recognized that the addition of water to POCl3 dramatically changes its phosphorylating properties although the identity of the active reagent so generated remains uncertain. We have developed a new method for studying the hydrolysis products of PCl5 or POCl3 generated in situ, using limited hydrolysis in 18-O water, followed by complete hydrolysis in normal water. These studies have provided information on the mechanism and kinetics of the in situ hydrolysis process and also revealed the presence of phosphorochloridic acid, H2PO3CL, a hitherto undetected hydrolytic intermediate and a likely candidate as the active phosphorylating reagent in partial hydrolysis mixtures.